1. Field of the Invention
The invention relates to a hydrokinetic torque converter with a bladed stator for use in an automotive vehicle driveline to establish a hydrokinetic torque flow path between an engine and transmission gearing.
2. Background Art
A typical hydrokinetic torque converter for use in an automotive vehicle driveline consists of three bladed elements: an impeller, a turbine and a reactor or stator. A converter of this type is described, for example, in a paper presented by V. J. Jandasek of Ford Motor Company at an L. Ray Buckendale Lecture in January, 1961.
The bladed elements of a hydrokinetic torque converter define a closed fluid flow circuit, the impeller being driven by the engine and the turbine being connected to torque input elements of a geared transmission. The reactor or stator is grounded to the transmission case by means of an overrunning clutch.
The impeller centrifugally pumps fluid to the turbine, which in turn absorbs the hydrokinetic energy of the fluid and by deflecting the fluid flow to its discharge area, the direction of flow in the torque circuit being reversed by the turbine blades. The stator provides the necessary torque reaction for the converter by re-directing the flow from the turbine in a forward direction. The impeller receives the re-directed fluid thereby creating an overall converter torque ratio.
As the turbine speed increases, a centrifugal head is created, which counteracts the centrifugal head of the impeller and reduces the absolute fluid flow velocity in the torque circuit, thereby reducing the reaction torque on the stator and reducing the torque transmitted by the over-running clutch to the case. Eventually, the torque on the reactor becomes zero as the turbine speed increases further. At this point, the converter ceases to multiply torque as it enters its fluid coupling stage. The transition from the converter stage to the fluid coupling stage is referred as the coupling point. Further increases in turbine speed will result in forward rotation of the reactor as the over-running clutch free wheels.
The impeller consists of an impeller housing, impeller blades and an inner impeller shroud. The blades typically are attached to the housing and shroud with tabs or by brazing. The impeller is connected to the engine crankshaft.
The turbine consists of a turbine inner shroud, turbine blades, and a turbine shell or outer shroud. The blades are attached to the shrouds. The turbine is drivably connected to the transmission gearing input shaft through a turbine shaft.
The stator consists of radially inward and radially outward stator shrouds, stator blades, and a stator hub. The stator blades, in a stator of the kind discussed in the paper of V. J. Jandasek, are formed with an airfoil-type profile. The stator may be manufactured using a casting method. If the casting dies are of the axial-pull type, the stator shrouds, stator blades and the hub would be formed as one integral part. If radial-pull dies are used in the manufacturing method, the radially outward stator shroud would be a separate part and would be attached to the radially outward margins of the blades by a special assembly process.
The stator is mounted on a transmission stator support shaft using an overrunning clutch or coupling as previously mentioned.
With a contemporary three-element torque converter, improved powertrain efficiency is achieved by using a mechanical converter lock up clutch, which comprises a piston plate, a clutch element and a coil spring damper installed between the turbine outer shroud and the converter impeller housing. This defines a bypass torque flow path from the engine to the transmission, which effects a direct mechanical drive for the vehicle powertrain.
In a three-element torque converter of this type, the input mechanical power from the engine is transferred through the fluid hydrodynamically due to the pumping action established by the impeller. As fluid is discharged from the impeller exit section and received by the turbine, the hydrokinetic energy of the fluid is transformed to mechanical energy. Because the angularity of the fluid flow vectors for a particle of fluid at the stator entrance section vary through a wide range of flow angles, a smooth and blunt airfoil-type profile for the leading edge of impeller blades typically is used to reduce the flow entrance losses and to improve converter performance. Such a profile, however, will reduce the effective flow area for the fluid transferred from the turbine exit section to the impeller entrance section through the stator blading. These counteractive effects must be considered in the stator blade design to achieve an optimum blade profile.
In the design of the profile for the stator blades, fundamental assumptions must be made. The performance results, following the initial design, can be varied using independent dynamometer data, but an initial theoretical analysis of the stator blade geometry using those assumptions will achieve an approximation of proposed blade data prior to dynamometer tests. Adjustments using information obtained during actual tests then can be used to modify the theoretical analysis.
One of the most fundamental assumptions that is made in a theoretical analysis of a stator blade profile is that the fluid circulating through the torus circuit of the converter follows a path that corresponds to the contour of the blades. It follows from this assumption that the fluid exit angle equals the exit angle of the blade itself. A relationship then can be established between the blade angles and the velocity vector components of a particle of fluid at a particular location in the torus circuit to obtain expressions for torque and torus flow velocity.
It is assumed also that the entire body of fluid flow particles in the torus circuit travels in the same general direction in such a manner that a tangent vector at any point on the blading is the velocity vector component of a moving body of fluid at that point. Known analytical techniques for determining blade geometry refer to this path of movement of a particle of fluid as a mean streamline. This mean streamline is that particular path over which it can be assumed that the entire mass of fluid flows. This is done to simplify the analysis.
One technique for locating the mean streamline includes computation of the root mean square distance from the axis of revolution to the inner and outer stator shrouds. This method has been proven by experience to be approximately correct. Typically, the mean streamline would tend to develop relatively close to the center of the torus. In the case of the stator, therefore, the streamline would be closer to the radially outward shroud than the radially inward shroud. Another suggested method used in earlier techniques includes designing the torus circuit cross section such that the torus area on the outer side of the streamline equals the torus area on the inner side. Obviously, such a streamline would fall closer to the radially inward stator shroud than the radially outward stator shroud. The former method may be preferred in most cases since principles of fluid mechanics will verify that streamlines in fluid flow around a corner tend to become more dense at a region closer to the center of curvature than at a region farther away due to local increases in pressure.
Another assumption, which is of relatively minor importance, is that there are no short circuiting losses as the particles of fluid traverse the torus circuit. Thus, the principle of continuity of flow can be applied without the necessity for using correction factors. Torus flow velocity then will vary at different points along the torus circuit only because of the variation in cross sectional areas along the torus circuit. The mass of fluid flowing per second at any point along the torus circuit may be expressed in terms of the flow velocity at one point, such as the flow velocity at the impeller flow exit area.
Vector diagrams of the motion of a particle fluid in the torus circuit are useful in an analysis of the stator blade geometry. These diagrams are two-dimensional representations that can simplify the mathematical procedure. This analysis is analogous to a so-called unwrapping of the circuit that makes a three-dimensional representation unnecessary. By unwrapping the circuit in an analysis of this kind, a continuous stator blade cascade is formed in a single plane.
A preliminary analysis of the kind discussed in the preceding paragraphs would not necessarily take into account all design factors that may affect the result. Some of those factors are the effect of surface roughness, the constriction of the passage due to large exit angles, the effect of viscosity, etc. A human element, therefore, should be introduced into the analysis based upon prior experience and empirical data to determine whether the flow values and the angularity of the stator blade determined analytically are reasonable.
Prior art U.S. Pat. No. 5,616,000 discloses a stator blade construction that is intended to avoid separation of the working fluid from the surface of the blade on the pressure side of the blade when the speed ratio is in a high-speed ratio range near the coupling point. The profile of the surface on the low pressure side of the blade is established by tracing out the envelope of a series of circles along the streamline, the radius of the circles following a specified function or relationship of characteristic parameters.
Prior art U.S. Pat. No. 6,003,311 discloses a stator blade with a modified fluid entrance section that is intended to improve inlet flow during high speed ratio operation. The profile of the blade surface on the high pressure side of the blade follows the contour of a streamline or a particle of fluid passing through the openings between the blades of the stator at a mid-position of the fluid flow and at the trailing blade tip. At the entrance section of the stator blade, the shape of the blade surface on the high pressure side of the blade is generally planar. This is intended to improve operating efficiency of the powertrain during idling of the engine.